Soldering is a low temperature, generally reversible, metallurgical joining process. Low temperature and reversibility are especially important in electronics applications because of the materials involved and the necessity for reworking and making engineering changes.
Solder joining is a wetting process followed by a chemical reaction. Molten solder wets selectively. The selective wettability of solders allow molten solder to be confined to desired sites. This is especially important in flip chip bonding, and in working with solder masks.
The soldering process can be accomplished as quickly as the wetting takes place, for example, on the order of a few seconds. This makes soldering particularly desirable for automated, high speed, high throughput processes.
Wettability is also a function of the materials to be joined, with Cu, Ni, Au, and Pd, as well as alloys rich in one or more of these metals, being particularly amenable to soldering.
The chemical reaction following wetting is between the molten solder and the joining metallurgy to form an intermetallic phase region at the interface. The intermetallic phases formed by solders in electronic packaging are stoichiometric compounds, typically binary compounds, and typically containing Sn if Sn is present in the solder alloy. When the base, pad, or land is Cu, and the solder alloy is rich is Sn, the intermetallic formed during soldering is Cu-Sn. Exemplary Cu-Sn binaries include Cu.sub.3 Sn and Cu.sub.6 Sn.sub.5.
Solder alloys are characterized by the melting temperature being a strong function of composition. While a pure metal is characterized by a single, invariant, melting temperature, the freezing and melting points of alloys are complex. The freezing point of an alloy is determined by the liquidus line. Above the liquidus line only a liquid phase or phases can exist. The melting point of an alloy is determined by the solidus line. Below the solidus line only a solid phase or phases can exist. In the region between these two lines, i.e., between the liquidus line and the solidus line, solid and liquid phases can co-exist.
The preferred soldering alloys are eutectics, that is, they are characterized by a eutectic point. The eutectic point is where the liquidus and solids lines meet. A concentration change in either direction from the eutectic results in an increase in the liquidus temperature.
The composition, and the quench rate, also determine the microstructure and the resulting mechanical properties of the solder joint. Thus, it is necessary to both carefully choose the solder composition and to carefully control the thermal exposure of the soldered joint.
A solder composition used in electronics fabrication must be wettable as a solder alloy, and have at least one component capable of forming an electrically conductive, thermally stable, non-brittle, plastic intermetallic with the pad or land metallurgy. For this reason, the most common solder alloys are lead based alloys, as Sn-Pb alloys.
Heretofore, Pb/Sn solders have been utilized for electronic applications. There have been many historical reasons for the wide spread use of Pb/Sn alloys. These historical reasons include the low solidus temperature of Pb/Sn solder alloys, the workability of Pb/Sn alloys and of the resulting Cu/Sn intermetallics (formed at the solder/Cu contact interface) over a wide temperature range, the adhesion of Cu/Sn intermetallics obtained from Pb/Sn alloys to Cu lands and pads, and the ready availability of process equipment and low cost adjuncts, as resins, fluxes, and solder masks, for Pb/Sn alloys.
The relatively low temperatures required for processing Pb/Sn solder alloys are particularly important when polymeric dielectrics are used in the fabrication of electronic packages. These polymers can degrade in high temperature assembly operations. Solder alloys which melt at relatively low temperatures can accommodate these polymeric substrates.
Additionally, semiconductor chips are subject to thermal diffusion and structural transformations at elevated temperatures. Low melting solders avoid these problems.
Especially important is the "softness" or plasticity of lead based solders. This softness or plasticity allows the solder to accommodate the mismatch in coefficients of thermal expansion between the bonded structures, for example the mismatch in coefficient of thermal of thermal expansion between a ceramic dielectric and a polymeric dielectric, or between a semiconductor chip and a ceramic or polymeric chip carrier or substrate.
However, lead is a toxic, heavy metal with a relatively high vapor pressure. Its use is disfavored, and a need exists for a replacement.
U.S. Pat. No. 4,806,309 to Stanley Tulman for TIN BASE LEAD-FREE SOLDER COMPOSITION CONTAINING BISMUTH, SILVER, AND ANTIMONY describes solder compositions containing from 90 to 95% Sn, from 3 to 5% Sb, from 1 to 4.5% Bi, and from 0.1 to 0.5% Ag. Tulman describes the use of Bi to lower the melting point of the solder to about 425 degrees F (218 degrees C).